diff --git a/hermes_cli/cli_modal_mixin.py b/hermes_cli/cli_modal_mixin.py
index c2c940da0d..44b1f466d7 100644
--- a/hermes_cli/cli_modal_mixin.py
+++ b/hermes_cli/cli_modal_mixin.py
@@ -549,18 +549,29 @@ class CLIModalMixin:
         flag = "bell_on_prompt" if prompt else "bell_on_complete"
         if not getattr(self, flag, False):
             return
+        from hermes_cli.terminal_notify import notification_sequence, notify as _terminal_notify
+        body = context or ("input needed" if prompt else "turn complete")
+        session_id = getattr(self, "session_id", "") or ""
+        app = getattr(self, "_app", None)
+        if app is not None and getattr(app, "_is_running", False):
+            # Agent thread. The loop thread may be mid-write of a 12 KB kitty pet frame that the tty
+            # drains ~1 KB at a time; a second writer on the same tty (/dev/tty, sys.stdout) splices
+            # in, the foreign ESC aborts the APC, and the terminal paints the rest of the payload as
+            # base64 at the input cursor. Serialize behind the renderer instead.
+            from hermes_cli.cli_terminal_mixin import _run_on_app_loop, _write_terminal_sequence
+            try:
+                seq = "\a" + notification_sequence(body, prompt=prompt, session_id=session_id, detail=detail)
+                _run_on_app_loop(app, lambda: _write_terminal_sequence(app, seq))
+            except Exception:
+                pass
+            return
         try:
             sys.stdout.write("\a")
             sys.stdout.flush()
         except Exception:
             pass
         try:
-            from hermes_cli.terminal_notify import notify as _terminal_notify
-            _terminal_notify(
-                context or ("input needed" if prompt else "turn complete"),
-                prompt=prompt,
-                session_id=getattr(self, "session_id", "") or "",
-                detail=detail)
+            _terminal_notify(body, prompt=prompt, session_id=session_id, detail=detail)
         except Exception:
             pass
 
diff --git a/hermes_cli/terminal_notify.py b/hermes_cli/terminal_notify.py
index 4b370ef8d3..100ed064e8 100644
--- a/hermes_cli/terminal_notify.py
+++ b/hermes_cli/terminal_notify.py
@@ -65,10 +65,16 @@ def warp_osc777(event: str, detail: str, session_id: str = "") -> str:
     return f"\x1b]777;notify;warp://cli-agent;{json.dumps(payload, separators=(',', ':'))}\x07"
 
 
-def notify(context: str, *, prompt: bool, session_id: str = "", detail: str = "") -> None:
-    """Emit OSC 9 (plus Warp OSC 777 when supported) for a blocking prompt or turn end."""
+def notification_sequence(context: str, *, prompt: bool, session_id: str = "", detail: str = "") -> str:
+    """OSC 9 (plus Warp OSC 777 when supported) for a blocking prompt or turn end."""
     seq = osc9(f"Hermes: {context}")
     if warp_supported():
         event = "permission_request" if prompt else "stop"
         seq += warp_osc777(event, detail or context, session_id)
-    _write_tty(seq)
+    return seq
+
+
+def notify(context: str, *, prompt: bool, session_id: str = "", detail: str = "") -> None:
+    """Emit the notification straight to the tty. Only for callers that do not own a running
+    prompt_toolkit app; inside the CLI, ``_ring_bell`` routes it through the app output instead."""
+    _write_tty(notification_sequence(context, prompt=prompt, session_id=session_id, detail=detail))
diff --git a/tests/hermes_cli/test_terminal_notify.py b/tests/hermes_cli/test_terminal_notify.py
index fce8b90d5e..e5a9092dc5 100644
--- a/tests/hermes_cli/test_terminal_notify.py
+++ b/tests/hermes_cli/test_terminal_notify.py
@@ -1,6 +1,10 @@
 """display.bell_on_prompt / bell_on_complete also drive OSC 9 + Warp OSC 777 via _ring_bell."""
 
+import io
 import json
+import sys
+
+import pytest
 
 from cli import HermesCLI
 from hermes_cli import terminal_notify
@@ -48,3 +52,45 @@ def test_warp_osc777_only_under_supported_warp_build(monkeypatch):
     # Not Warp at all → OSC 9 only.
     not_warp = dict(_WARP_OK, TERM_PROGRAM="ghostty")
     assert prefix not in _ring(monkeypatch, flag_on=True, env=not_warp, context="approval")
+
+
+def test_running_app_gets_bell_and_osc9_on_its_loop_never_a_second_tty_writer(monkeypatch):
+    """With the prompt_toolkit app live, the notification must reach the tty through the app's
+    output ON THE APP LOOP. A parallel /dev/tty or sys.stdout write from the agent thread splices
+    into an in-flight kitty pet frame and the terminal paints the frame's base64 as text."""
+    for key in _WARP_OK:
+        monkeypatch.delenv(key, raising=False)
+    monkeypatch.setattr(terminal_notify, "_write_tty", lambda seq: pytest.fail(f"stray tty write: {seq!r}"))
+    fake_stdout = io.StringIO()
+    monkeypatch.setattr(sys, "stdout", fake_stdout)
+
+    class _Output:
+        raw = []
+
+        def write_raw(self, data):
+            self.raw.append(data)
+
+        def flush(self):
+            self.raw.append("<flush>")
+
+    class _Loop:
+        queued = []
+
+        def call_soon_threadsafe(self, fn):
+            self.queued.append(fn)
+
+    class _App:
+        _is_running = True
+        loop = _Loop()
+        output = _Output()
+
+    cli = HermesCLI.__new__(HermesCLI)
+    cli.bell_on_complete = True
+    cli.session_id = "sess-1"
+    cli._app = _App()
+    cli._ring_bell(context="turn complete")
+    # Nothing touched the tty from the calling thread; the write is queued for the loop.
+    assert _Output.raw == [] and fake_stdout.getvalue() == ""
+    assert len(_Loop.queued) == 1
+    _Loop.queued[0]()
+    assert _Output.raw == ["\a\x1b]9;Hermes: turn complete\x07", "<flush>"]